The Modular Dynamics Tester (MDT) is an instrument used to acquire aliquots of reservoir fluid for analyses and transportation. The reservoir fluid is drawn into the MDT through a probe in-contact with the bore-hole wall by reducing the pressure within the MDT tubular, which contains bore-hole fluid, from the pressure of the formation. The pressure reduction is generated by a positive displacement pump operated by hydraulic fluid. When the fluid within the tubular is, for all intensive purposes, free of drilling fluid, as determined by the interpretation of independent measurements on the flow-line, the reservoir fluid is directed into the sample bottle. The position of the piston within the sample bottle and thus the intake of fluid are not currently determined.
Department of Transportation (“DOT”) certified bottles are used to move fluid from the well-site to the laboratory. These may contain hydraulic fluid and, in one case, a nitrogen buffer to significantly reduce the pressure reduction arising from a temperature decrease. Ideally, for any replacement reservoir fluid sampling instrument, the sample collection vessel will include the N2(g) buffer and be DOT certified and for the remainder of this the description will assume this is so.
Current apparatus do not accurately determine piston position during fluid withdrawal activities. The methods provided and the apparatus for conventional systems do not provide sufficient resolution to allow operators control to the desired level.